[Special Papers in Palaeontology 77, 2007, pp. 207–222] WHAT PNEUMATICITY TELLS US ABOUT ‘PROSAUROPODS’, AND VICE VERSA by MATHEW WEDEL University of California Museum of Paleontology and Department of Integrative Biology, 1101 Valley Life Sciences Building, Berkeley, CA 94720-4780, USA; e-mail: [email protected] Typescript received 15 February 2006; accepted in revised form 24 October 2006 Abstract: Pneumatic (air-filled) bones are an important fea- may have evolved once (at the base of Ornithodira), twice ture of the postcranial skeleton in pterosaurs, theropods and (independently in pterosaurs and saurischians) or three times sauropods. However, there is no unambiguous evidence for (independently in pterosaurs, theropods and sauropods). Skel- postcranial pneumaticity in basal sauropodomorphs and even etal pneumaticity appears to be more evolutionarily malleable the ambiguous evidence is scant. Patterns of skeletal pneumati- than the air sacs and diverticula that produce it. The evolution zation in early sauropods and theropods suggest that basal sau- of air sacs probably pre-dated the appearance of skeletal pneu- rischians had cervical air sacs like those of birds. Furthermore, maticity in ornithodirans. patterns of pneumaticity in most pterosaurs, theropods and sauropods are diagnostic for abdominal air sacs. The air sacs Key words: Prosauropoda, Sauropodomorpha, Saurischia, necessary for flow-through lung ventilation like that of birds Ornithodira, pneumaticity, air sacs, diverticula. Pneumaticity is a prominent feature of the postcra- However, the monophyly or paraphyly of the group of nial skeleton in theropod and sauropod dinosaurs. In con- taxa traditionally called prosauropods is not critical to the trast, there is little evidence for postcranial pneumaticity in purposes of this paper. What is important is that all tra- basal sauropodomorphs (informally referred to as ‘prosaur- ditional ‘prosauropods’ have two things in common: they opods’ in this paper), although from time to time some lack unequivocal evidence of pneumatic cavities in their aspects of prosauropod osteology have been posited as evi- vertebrae and ribs, and they are phylogenetically brack- dence of pneumaticity (Britt 1997) or compared with eted by sauropods and theropods (Text-fig. 1). unequivocal pneumatic structures in sauropods (Yates 2003; Galton and Upchurch 2004). My goals in this paper Institutional abbreviations. BMNH, The Natural History are to review the evidence for postcranial skeletal pneuma- Museum, London; CM, Carnegie Museum of Natural His- ticity (PSP) in prosauropods and to discuss the origin of air tory, Pittsburgh, USA; FMNH, Field Museum of Natural History, sacs and pneumaticity in early dinosaurs and their relatives. Chicago, USA; MSM, Mesa Southwest Museum, Mesa, USA; Prosauropod taxonomy is currently in a state of flux, OMNH, Oklahoma Museum of Natural History, Norman, USA; SMNS, Staatliches Museum fur Natu¨rkunde, Stuttgart, Germany. as other papers in this volume attest (Sereno 2007; Upchurch et al. 2007; Yates 2007). Prosauropods were Anatomical abbreviations. ACDL, anterior centrodiapophyseal traditionally considered a paraphyletic assemblage that lamina; AL, accessory lamina; AVF, anteroventral fossa; NAF, gave rise to sauropods. Sereno (1998) recovered a mono- neural arch fossa; PCDL, posterior centrodiapophyseal lamina; phyletic Prosauropoda, defined this clade (anchored upon PDF, posterodorsal fossa; PODL, postzygodiapophyseal lamina; Plateosaurus) as a monophyletic sister taxon to Sauropoda PPDL, paradiapophyseal lamina; PRDL, prezygodiapophyseal and united the two in a node-based Sauropodomorpha. A lamina; SPOL, spinopostzygapophyseal lamina; SPRL, spinoprezy- similar phylogenetic hypothesis was described by Galton gapophyseal lamina (lamina abbreviations after Wilson 1999). and Upchurch (2004). However, other recent phylogenet- ic analyses (Yates 2003, 2004; Yates and Kitching 2003) have found that some prosauropods are closer to Salta- POSTCRANIAL PNEUMATICITY IN saurus than to Plateosaurus; thus, under current phylo- THEROPOD AND SAUROPOD genetic definitions they should be regarded as basal DINOSAURS sauropods. Some other taxa (e.g. Saturnalia) lie outside Sauropodomorpha as defined by Sereno (1998) altogether Before examining the evidence for PSP in ‘prosauropods’, (Yates 2003, 2004; Yates and Kitching 2003; Langer 2004). I will review the conditions present in other saurischian ª The Palaeontological Association 207 208 SPECIAL PAPERS IN PALAEONTOLOGY, 77 skin. Where a diverticulum comes into contact with a bone, it may (but does not always) induce bone resorp- tion, which can produce pneumatic tracks, fossae or for- amina. If resorption of the cortex produces a foramen, the diverticulum may enter the medullary space and replace the existing internal structure with a series of air- filled chambers of varying complexity. The best descrip- tion of this process is provided by Bremer (1940). The extent of PSP varies in different avian clades. Almost any postcranial bones can become pneumatized; in large soaring birds such as pelicans, almost the entire skeleton is pneumatic, including the distal limb elements (O’Connor 2004). Although many large volant and flight- less birds have highly pneumatic skeletons, the correlation between body size and the extent of PSP in birds is weak (O’Connor 2004). PSP tends to be reduced or absent in diving birds (Gier 1952; O’Connor 2004). Different parts of the skeleton become pneumatized by diverticula of dif- ferent air sacs in extant birds (Table 1); this is important because it allows us to make inferences regarding the evo- lution of air sacs in fossil taxa. PSP in non-avian thero- TEXT-FIG. 1. A phylogeny of archosaurs showing the pods generally follows the avian model (Britt 1993, 1997; evolution of postcranial skeletal pneumaticity and air sacs. O’Connor and Claessens 2005; O’Connor 2006). Patterns Thecodontosaurus is shown as having limited postcranial of pneumatization along the vertebral column indicate pneumaticity. The evidence for this is ambiguous; see text for that both anterior and posterior air sacs (presumably cer- discussion. Based on the phylogenetic framework of Brochu vical and abdominal) had evolved by the time of the cer- (2001) and Yates (2003). atosaur-tetanuran divergence (O’Connor and Claessens 2005). dinosaurs. The sister taxon of Sauropodomorpha is Ther- Fossae are present in the presacral vertebrae of basal opoda; consequently, ‘prosauropods’ are phylogenetically sauropods such as Shunosaurus and Barapasaurus (Britt bracketed in part by birds, the only clade of extant verte- 1993; Wilson and Sereno 1998). These fossae are similar brates with extensive PSP. The relationship between the to the unequivocally pneumatic foramina and camerae of respiratory system and pneumatic postcranial bones in more derived sauropods, both in their position on indi- birds has been described many times (e.g. Mu¨ller 1908; vidual vertebrae and in their distribution along the ver- King 1966; Duncker 1971; O’Connor 2004), and is briefly tebral column, and because of these similarities they have summarized here. The relatively small, constant-volume, usually been regarded as pneumatic in origin (Britt 1993, unidirectional flow-through lungs of birds are ventilated 1997; Wedel 2003a). However, similar fossae are present by the attached air sacs, which are large, flexible and in other tetrapods that lack PSP, so the presence of fossae devoid of parenchymal tissue. The lungs and air sacs also alone is at best equivocal evidence for PSP (O’Connor produce air-filled tubes called diverticula that pass 2006; see below). The vertebrae of more derived sauro- between the viscera, between the muscles, and under the pods have foramina that communicate with large internal TABLE 1. Parts of the postcranial skeleton that are pneumatized by diverticula of different parts of the respiratory system in extant birds. Pneumaticity varies widely within populations and clades, and not all elements are pneumatized in all taxa (based on Duncker 1971 and O’Connor 2004). Respiratory structure Skeletal elements Lung (parenchymal portion) Adjacent thoracic vertebrae and ribs Clavicular air sac Sternum, sternal ribs, pectoral girdle and humerus Cervical air sac Cervical and anterior thoracic vertebrae and associated ribs Anterior thoracic air sac Sternal ribs Posterior thoracic air sac (none reported) Abdominal air sac Posterior thoracic, synsacral and caudal vertebrae, pelvic girdle and femur Subcutaneous diverticula Distal limb elements WEDEL: WHAT PNEUMATICITY TELLS US ABOUT ‘PROSAUROPODS’ 209 chambers; the combination of foramina and large internal shown that the ilial chambers are connected to foramina, chambers is an unambiguous indicator of PSP (O’Connor which are necessary for pneumatization to occur (see 2006). There is a general trend in sauropod evolution for O’Connor 2006). PSP to spread posteriorly along the vertebral column, albeit to different extents in different clades and with a few reversals (Wedel 2003b; Text-fig. 2). In both sauro- EVIDENCE OF PNEUMATICITY IN pods and theropods, fossae in basal forms were replaced ‘PROSAUROPODS’ by large-chambered (camerate) vertebrae and eventually small-chambered (camellate) vertebrae in more derived Historically, postcranial pneumaticity in ‘prosauropods’ taxa (Britt 1993, 1997; Wedel 2003a). has received little attention, which is to be expected given The evolution of PSP in sauropods mirrors in detail the paucity of available evidence. Janensch (1947)